Yeast Attenuation

If you were yeast, attenuation would simply be how much of your restaurant meal you actually consumed:

100 percent attenuation means you cleaned your plate.

75 percent attenuation means you took home about a quarter of your dinner in a doggy bag.

50 percent attenuation means you probably should have split an entrée with your dining partner.

0 percent attenuation means you lost your appetite after ordering.

A highly attenuative yeast strain consumes more wort sugars than a strain that exhibits lesser attenuation. While each yeast is unique, Belgian strains tend to attenuate a lot, American strains moderately, and English strains less enthusiastically.

But we have to be careful: Attenuation is how much sugar the yeast eats, but this isn’t what we measure. Brewers use the hydrometer to measure a beer’s original and final gravities, which, in turn, indicate the density of a solution. Wort density increases with the amount of sugar dissolved in it, so density is an accurate proxy for the real extract of a sugar solution.

Real extract is what you would get if you could somehow physically separate out all of the wort sugars into a little pile, weigh the pile, and then compare that to the total weight of the wort in the fermentor. This isn’t practical, so we use density (gravity) as a reliable indicator of what that number would be.

However, when the miracle of fermentation transforms wort into beer, the end result isn’t just a sugar solution, but rather a mix of sugar, water, ethanol (alcohol), and negligible amounts of proteins, esters, phenols, and other substances. Alcohol is less dense than water, so a hydrometer can’t accurately measure the amount of residual sugar remaining in solution: The alcohol throws off the hydrometer reading. Ergo, comparing original gravity (OG) and final gravity (FG) doesn’t accurately indicate the degree of real attenuation.

Practically speaking, though, this doesn’t matter because brewers almost always talk in terms of apparent attenuation (AA), which is related to the difference between the observed gravities.

AA = (OG – FG) ÷ (OG – 1)

It’s called apparent attenuation because it’s based on observed quantities instead of physically accurate ones. It’s still a reliable indicator of how much sugar the yeast consumed, just not in the strictest academic sense. If you create wort with an original gravity of 1.050, and your selected yeast strain ferments it down to an final gravity of 1.010, then the apparent attenuation isAA = (1.050 – 1.010) ÷ (1.050 – 1) = 0.8 = 80%

Now the real attenuation in this case (that is, the actual percentage of sugars consumed) is more like 65 percent, but because alcohol is less dense than sugar water, the hydrometer makes it appear as if attenuation is greater than it really is.Unless you are a professional brewer or a chemist, apparent attenuation is all you really need to know. When someone boasts that his saison hit 90 percent attenuation, he is almost certainly referring to apparent attenuation.

Conveniently, the major yeast suppliers all express attenuation in terms of the apparent variety. This means that you can use their numbers directly when formulating your recipe. In the example above, if you were to prepare a wort with an original gravity of 1.050 and ferment it with a strain whose published apparent attenuation is 80 percent, then you could reasonably assume that your beer’s final gravity would end up around 1.010.